Apply percent sign escaping to op(), custom_op()

Apply percent sign escaping to op(), custom_op()

Fixed bug where the "percent escaping" feature that occurs with dialects
that use the "format" or "pyformat" bound parameter styles was not enabled
for the :meth:`.Operations.op` and :meth:`.Operations.custom_op` methods,
for custom operators that use percent signs. The percent sign will now be
automatically doubled based on the paramstyle as necessary.

Fixes: #6016
Change-Id: I285c5fc082481c2ee989edf1b02a83a6087ea26a